1. Field of the Invention
The present invention relates to a multiple-power IC device such as a multiple-power semiconductor integrated circuit which has an input buffer circuit compatible with a plurality of powers.
2. Description of the Prior Art
In general, the input terminal of a semiconductor IC device has an input buffer circuit which functions as an interface. This input buffer allows signals to be sent from the input terminal to the internal circuit of a semiconductor IC device based on a threshold according to the voltage supplied to the input buffer circuit.
This type of IC device is usually built on a printed circuit board. For a printed circuit board to be used as a TTL logic circuit, two power voltage levels, 3V and 5V, are supplied in most cases. And, depending upon the environment in which this IC device is used, a 3V or 5V power level is selected for use in the input buffer circuit of the IC device built on the circuit board.
In many cases, a Complementary Metal Oxide Semiconductor (CMOS) buffer is used as a multi-voltage-power input buffer. Upon receiving an input signal, this CMOS buffer sends an output signal to the internal circuit according to its threshold. This threshold is determined by two factors: one is the ratio between the p-channel MOSFET gate width and the n-channel MOSFET gate width of the CMOS buffer and the other is the voltage of the power supplied to the buffer. And, by matching the threshold of this buffer to the medium between the L (low) level and the H (high) level of the input signal, a sufficient noise margin is provided for each of both the L level and the H level of the input signal.
For example, when a TTL-level signal with an amplitude of about 3V is sent to a buffer with a power voltage of 5V and when the threshold of the buffer is 1.5V which is about the medium of the input signal level, it is possible for the buffer to match the threshold to the medium of the L level and H level of the input signal. In this case, a sufficient margin is provided for each of the L level noise and the H level noise.
However, when a 3V driving power is supplied to a buffer whose threshold is 1.5V for a 5V power voltage, the threshold varies; for example, it is lowered to 0.9V. This makes the L level noise margin for the input signal extremely small. Thus, the threshold of the conventional multiple-power IC circuit varies according to the driving power of the input buffer, resulting in a variation in the input signal noise margin.
It is an object of the present invention to provide a multiple-power IC device in which the threshold of the input buffer circuit does not vary even if the input buffer circuit voltage is selected from a plurality of voltages. It is another object of the present invention to provide a multiple-power IC device in which the input signal noise margin does not vary according to the power voltage.